Posted by: Dan | January 5, 2010

Paleoclimate Reconstructions

Here on my small corner of the interwebs, I occasionally still have climate change contrarians leave comments. I’m tired of it. But I do want to put up a single blog post on one crucial part of this topic to point to whenever someone claims that the work of Michael Mann et al. is somehow part of a conspiracy. Here it is.

Figure 6.10b (IPCC4, WG1)



So, about this “Hockey Stick,” first published in 1998 by Mann, Bradley and Hughes (aka the MBH analysis). Two major inquiries were done in the political sphere, and those are the NRC Report (2006) and the Wegman report (2006). Both are available online, and both found that while there were statistical shortcomings in the MBH analysis, but concluded that they were small in effect. The NRC report for instance summarizes its main findings as follows:

  • The instrumentally measured warming of about 0.6 °C (1.1 °F) during the 20th century is also reflected in borehole temperature measurements, the retreat of glaciers, and other observational evidence, and can be simulated with climate models.
  • Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the “Medieval Warm Period”) and a relatively cold period (or “Little Ice Age”) centered around 1700. The existence and extent of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.
  • It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies.
  • Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. The uncertainties associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified.
  • Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data and the methods used to analyze and combine them are larger than during more recent time periods.

That amounts to a near-complete vindication of the MBH analysis. Alternatively, the Wegman Report, though critical of the MBH analysis, also (inadvertently) vindicated the MBH analysis.

But the MBH analysis was hardly the only climate reconstruction of recent millenia, it’s only the one at the center of the controversy. There’s a substantial list of studies published in the past twelve years, using various data sets and various statistical techniques. I won’t go into all of them, but they all show pretty much the same thing.

The MBH analysis and a lot of the other reconstructions rely on one type of proxy data (i.e., tree-rings), even if they use tree-ring data from different regions and are analyzed in with several statistical techniques. What about other proxy temperature data — what do they show compared to the tree-ring proxy data published by Mann and his peers?

The original MBH analysis, Figure5b

More complete, from N. hemisphere tree-rings (Briffa, 2000)

Based on bore-holes (Mann and Jones, 2003)

Based on glacial retreat (Oerlemans, 2005)

Multi-proxy assemblage (Moberg et al., 2005)

Note that they all show basically the same phenomenon, corroborating each other’s overall findings. The last one, from Moberg et al. (2005) is particularly significant, because it stands to reason that the findings of any one proxy source will be limited in data and perhaps have artifacts that are due to the proxy source and not reflective of the actual temperatures. Moberg and coworkers attempted to correct for this by smoothing out the data, showing only the patterns recurring in completely different records of what the temperatures might have been. They used reconstructions of ice melt and sedimentary deposition, pollen, corals and shells (Mg/Ca variation), sedimented diatoms, bore-hole measurements, stalagmite layer thickness, and tree-ring thickness. A more complete discussion can be found in Chapter 6.6 of the IPCC report (2007).

The nay-sayers still point out that there was a relatively warm period in the early centuries of the last millennium, and they’d be correct. There was. So much so that it is not quite correct to say that the 20th century was the warmest of those thousand years on the whole. Scientists aren’t claiming that however. The concern is the rate of warming, which was at such a rapid rate in the past thirty or so years. This is hinted at in the graph at the top of the page, where the various proxy data that I just got done showing are combined into one graph with modern instrumental data added in (Dark black line). But let’s zoom into just the actual instrumental record (the last 150 years)…

The instrumental record (1850 to 2005)

And what’s more is that there is reason to conclude that this warming trend of recent decades is not about to stop. I’ve previously put up a blog post explaining the history of greenhouse gases, and how advances in chemistry enabled the first predictions of global warming. Predictions like that of Svante Arrhenius (1896), who suggested that a 40% increase or decrease in the atmospheric abundance of the trace gas CO2 might trigger the glacial advances and retreats. A century later, it would be found that CO2 did indeed vary by this amount between glacial and interglacial periods.

Variations of deuterium (δD; black), a proxy for local temperature, and the atmospheric concentrations of the greenhouse gases

It’s important to note here however that, yes, if you look closely the levels of CO2 and the other greenhouse gases do not appear to initiate warming, but follow warming instead. Nevertheless, there is a strong positive correlation between CO2 and temperature in past ice ages, which suggests that warming somehow causes the release of greenhouse gases from carbon sinks, and the greenhouse gases enhanced warming, causing more carbon release, causing more warming, etc.

It is also important to note that in the past, release of greenhouse gases from carbon sinks was entirely natural (i.e., not anthropogenic). Now however, it’s not disputed that we dig up and burn petroleum, coal and natural gas. Nor is it disputed that we have in the 20th century greatly reduced the extent of the world’s forests. These are two major carbon sinks that aren’t dissolving into the atmosphere on their own. We’re doing that.

And just as CO2 co-varied with average global temperature before human civilization took root, so too will they co-vary with the post-industrial world.

End of story.

Explore Further:

  • Check out the Paleoclimate Reconstruction Challenge, and test a set of realistic pseudo-proxy series and calibration “instrumental data” to directly address concerns regarding the validity of climate reconstructions.

References:

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